mon: Monitor: StoreConverter: clearer debug message on 'needs_conversion()'

mon: Monitor: StoreConverter: clearer debug message on 'needs_conversion()'

The previous debug message outputted the function's name, as often our
functions do.  This was however a source of bewilderment, as users would
see those in logs and think their stores would need conversion.  Changing
this message is trivial enough and it will make ceph users happier log
readers.

Backport: cuttlefish
Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>